1. Field of the Invention
This invention relates to a noise cancellor, and in particular to a noise cancellor for actively canceling noises at an object point.
2. Description of the Related Art
Driving devices such as rotating machines, except for particular devices, generate noise when they are operating. The noise bring about various adverse influences on the environment. Generally, however, it is extremely difficult to obtain noiseless driving devices
Conventionally, there has been developed a noise cancellor for reducing noise at a specific place by using an acoustic technique. With this noise cancellor sound waves having reverse phases to and equal magnitudes to those of the noise at the specific place are artificially produced and are caused to interfere with the noise, thereby to actively cancel the noise at the specific place.
For example, in order to prevent that noise generated from a driving device as a noise source in a chamber leak out through the aperture of the chamber, it is possible to cancel the noises at the aperture, i.e., at an object point, by using this noise cancellor. In this case, the noise cancellor is generally constructed such that the noise generated by the driving device are detected by a receiver such as a microphone provided in the chamber and are converted into electric signals which are inputted to an arithmetic unit through an amplifier and an A/D converter. The signals output from the arithmetic unit are inputted through a D/A convertor to a sound generator such as a speaker provided near the aperture for producing required sound waves.
Let it be assumed that the noise generated by the driving device be S1, the sounds produced by the speaker be S2, the noise detected by the microphone be R1, the noise at the object point be R2, and the transfer functions between the driving device and the microphone, the driving device and the object point, the speaker and the microphone, and the speaker and the object point be T11, T12, T21 and T22, respectively, the following equation of a two-input two-output system is obtained: ##EQU1##
Since the noise cancellor is intended to make the sound level be zero, R2 can be set to be zero. Therefore, the following equation is obtained. EQU S2=(R1.multidot.T12)/(T12.multidot.T21-T11.multidot.T22) (2)
As understood from Eq. 2, if the sounds S2, which is obtained by multiplying the noise R1 detected by the microphone by a filter factor h, may be produced from the speaker, it is possible to make R2 be 0, where EQU h=T12/(T12.multidot.T21-T11.multidot.T22) (3)
Therefore, when the filter factor series (impulse responses) for minimizing the noise at the aperture of the chamber is calculated and stored in the arithmetic unit of the noise cancellor, the optimum S2 can be obtained from the following equation: EQU S2=R1.multidot.h (4)
Two noise canceling methods are considered when Eq. (4) is used.
With one method, time series signals obtained from the microphone are converted by means of Fourier transform to obtain frequency domain signals and the obtained signals are multiplied by transfer functions of the frequency domain designation. Thereafter, the resultant signals are converted again to time series signal by means of inverse Fourier transform, and these new time series signals are input to the speaker to produce sounds.
With this method, it is difficult to produce control sounds by the speaker at real time, because the signals are processed in batch. Since, however, a driving device such as a rotating machine repeatedly generates sounds having substantially the same waves, noise can be canceled by adjusting the timing of producing control signals in accordance with trigger signals which synchronize with the rotation of the rotating machine.
With the other method, transfer functions are converted to so called filter factor series (impulse responses) by means of inverse Fourier transformation. Further, time series data to be inputted to the speaker is obtained by convoluting the filter factor series and the time series data which are detected through the microphone. This second method is called FIR filter system, FIR being the abbreviation of Finite Impulse Response, and produces control sounds at real time.
With the second method, the control sounds are given by the following equation: ##EQU2## where h(i) is a filter factor series, X(n-1) is a closest sample datum of the i'th input signal, M is a tap number, i is a tap factor number, and S2(n) is the n'th output datum.
When both methods are used, noise at the aperture of the chamber can be actively canceled, and thus the noise generated by the driving device in the chamber can be prevented from leaking out of the chamber through the aperture.
With the conventional noise cancellors, however, the transfer functions from which the filter factor series are calculated are not always constant. In other words, the transfer functions vary according to the temperature change in the transmission paths of the sound, the change in the output characteristics of the speaker, the change in the characteristics of the driving device, and the like. For example, when the temperature in the chamber rises by heat generated from the driving device, the speed of sound changes, and this speed change varies the acoustic transfer functions. Further, when the speaker is continuously energized, the temperature of the coils of the speaker becomes higher and its resistance changes, whereby the output of the speaker and the transfer functions vary. If the noise generating positions of the driving device vary in the course of the operation of the device, the acoustic transfer functions also vary. Such variation of the transfer functions reduces effect of noise cancelation at the object point. In order to carry out effective noise cancelation, therefore, it is necessary to alter the value of the filter factor series according to the change of the transfer functions.
For the purpose of overcoming the above problem, recently a noise cancellor has been developed which is provided with an adaptive control function. In this cancellor, another microphone is arranged at the object point, and the filter factor series is automatically altered so that the outputs from the microphone become zero. The filter factor series of the noise cancellor having this control function is changed at constant time intervals according the following equation: EQU h(i) new=h(i) old+Ke X(n-i) (6)
where h(i) new is the i'th FIR filter factor after the alteration, h(i) old is the i'th factor before the alteration, K is a constant defining the alteration ratio of h, e is an error signal which is detected by the microphone at the object point, and X(n-i) is a closest sample datum of the i'th input signal.
However, this noise cancellor is encountered with the problems set forth below.
With this noise cancellor, the filter factor series is changed at constant time intervals while K is kept constant. The reason why K is kept constant is that the standard of changing K is not clear. However, if K is always kept constant, the following problems occur. When the time constant of the change of the physical factors, which determine the transfer function, is substantially identical to the time constant of the change of the filter factor h determined by the value of K, resonance occurs. Further, when the time constant of the change of the filter factor h depending on K is larger than that of the physical factors, control cannot be performed in accordance with the change of the physical factors. If the time constant of the change of h is rendered very small by increasing the value of K, the robustness of the control system is reduced. When it is known in advance that the change of the physical factors is slow, it is necessary that the value of K be very small. Very small K, however, leads to omission of bits or the like when signals are processed. Accordingly it is very difficult to select the value of K.
The disadvantages occurring from the alteration of the filter factor series at constant time intervals are as follows:
When the filter factors are altered too often by rendering the time interval too short, the robustness of the control system is reduced. On the contrary, when the frequency of the alteration is rendered small by making the time interval long, the control cannot follow to the change of the transfer function.
As described above, with the noise cancellor having an adaptive control function, the frequency of the change of the filter factor series, that is, the control convergence ratio of the control system is always constant, whereby the stability and the convergence may deteriorate, depending on the operation conditions of the noise cancellor and the driving device.
Further, with the conventional noise cancellors, when external noise propagates to the object point after the adjustment of the filter factor series has been finished and the convergence of control has been attained, the cancellors malfunction, changing the filter factors with the result that the complete noise reduction cannot be attained. Far from that, surplus sounds are produced at the object point.